Melt butter in 10-inch skillet until sizzling; add mushrooms and onion.,Cook over medium heat until tender (3 to 4 minutes).,Beat eggs in large bowl until frothy.,Pour into skillet.,Gently stir egg mixture over medium heat to cook evenly on bottom (3 to 4 minutes).,As egg mixture sets, lift edges with spatula to allow uncooked portion to flow underneath.,______,Cover; continue cooking until eggs are set (4 to 5 minutes).,Sprinkle with cheese; cut into wedges.